As a continuously variable transmission for a car, as shown in FIG. 11, there is known a transmission (a chain type continuously variable transmission) which comprises: a drive pulley (2) disposed on the engine side and including a fixed sheave (2a) and a movable sheave (2b); a driven pulley (3) disposed on the drive wheel side and including a fixed sheave (3b) and a movable sheave (3a); and, an endless shaped power transmission chain (1) wound around the drive and driven pulleys, wherein the movable sheaves (2b) and (3a) are made to move toward and away from the fixed sheaves (2a) and (3b) using an oil pressure actuator to thereby clamp the chain (1) with the oil pressure of the actuator, and this clamping force is used to generate a contact load between the pulleys (2), (3) and chain (1), whereby torque is transmitted due to a frictional force generated between the contact portions of the pulleys and chain.
As a power transmission chain which is one of winding members for use in a continuously variable transmission, in the patent reference 1, there is proposed a power transmission chain which includes: a plurality of links each including front and rear insertion portions through which pins are inserted; and, a plurality of first and second pins for connecting the links adjoining each other in the width direction of the chain so as to be bent in the longitudinal direction thereof and also that the front insertion portion of one link and the rear insertion portion of the other link can correspond to each other, wherein the first pin fixed to the front insertion portion of one link and movably fitted with the rear insertion portion of the other link and the second pin movably fitted with the front insertion portion of one link and fixed to the rear insertion portion of the other link are moved relative to each other in a rolling contact manner, whereby the two links can be bent in the longitudinal direction thereof.
In this type of power transmission chain, in order to enhance the durability of the chain, in the manufacturing process thereof, a tension is previously applied to the chain (application of a pre-tension) to thereby apply a proper residual compression stress to the link.
Also, in this type of continuously variable transmission, it is necessary to know the peripheral length of the chain with high precision. Specifically, the conical shape of the pulley is measured to estimate a contact diameter D, and there is obtained the peripheral length=πD+2L using a center distance L between the pulleys.
However, as regards the contact diameter D used in the above calculation of the peripheral length, since the contact ellipse area between the pin and pulley has a width in the diameter direction, the calculation of the peripheral length using the center value of the contact ellipse area finds it difficult to obtain sufficient measuring precision. This problem is found not only in the chain type continuously variable transmission disclosed in the patent reference 2 but also in a belt type continuously variable transmission.    Patent Reference 1: Japanese Patent Publication JP 2006-102784A    Patent Reference 2: Japanese Patent Publication JP 2005-233275A